1. Field of the Invention
The invention relates to radiation curable acrylic resin compositions and more particularly relates to ultraviolet curable polyacryl polyester coating compositions including flexible printing ink compositions and intermediate compositions therefor.
2. Brief Description of the Prior Art
In the prior art, most coating compositions, particularly of resinous materials were applied from a solvent solution, particularly organic solvents. The methods of application included brushing or spraying of a solvent solution, or dipping of the article into the coating solution. In each case a problem arose in that some means was generally necessary for reclaiming or disposing of the solvent which remained after or was stripped during the coating operation. The means generally involved substantial capital expenditures and added operating expense.
The problem of recovering solvents is particularly significant in printing operations. Many printing inks, such as those used for lithography, have heretofore been based on organic solvents. Following application of the solvated ink to the paper, the ink is set by heating which causes the evaporation of the organic solvent used to carry the ink base to the paper, sometimes with a concomitant cross-linking of the applied organic resinous material. If expensive solvent recovery equipment is not employed, then the solvent is evaporated to the air, adding to air pollution. With the radiation curable ink and coating vehicles of the present invention, no solvents are required and potential air pollution problems are obviated.
Solventless coating materials have been previously described; see for example, U.S. Pat. Nos. 3,551,235; 3,551,246; 3,551,311; and 3,616,364. Radiation curable ink and coating vehicles are generally liquid monomers or solutions of prepolymers dissolved in monomers, all of which copolymerize and solidify quickly when exposed to high energy radiation. In particular, the types of radiation employed are ultraviolet light and electron beam radiation. In addition to avoiding atmospheric pollution associated with solvent evaporation, radiation curing of ink and coating compositions requires little or no heat application. Thus, heat sensitive substrates, such as paper, plastics and wood, may be coated. Because of the speed of cure of a properly formulated radiation curable material, faster curing speeds may also be obtained, resulting in reduced operating costs. In addition, as cure only takes place where radiation has contacted the coating vehicle, clean-up problems are reduced as the unexposed, uncross-linked material may easily be removed from the remainder of the equipment or material being coated.
While some of the prior art radiation curing systems provided adequately rapid cure, they did not provide film flexibility which is required for many uses, particularly printing inks. The cured coatings of the present invention are highly flexible and therefore particularly useful in printing. In addition, the prior art materials provided a significant problem when employed in printing applications. In the printing operation there is a significant amount of paper waste. This comes from trimmed ends, misprints, etc. In order to operate economically, and to generally avoid waste of natural resources, it is necessary to recover and recycle this paper. In recovery, the paper is usually subjected to caustic treatment as in an original paper making process. The de-inking treatment must remove the ink which has been applied in order to produce an acceptable grade of paper in the recycling operation. The majority of prior art solventless, radiation cured coatings cannot be removed by this basic treatment, thus reducing the economics and desirability of their use. The cured coatings prepared from the compositions of this invention are readily removed from paper by caustic treatments.
Prior to this invention, acrylates of castor oil were disclosed in U.S. Pat. No. 3,010,925. These acrylates are curable by radiation but only extremely slowly.